87 Entrainment in laboratory analogues of cumulus and stratocumulus clouds tops

Monday, 7 July 2014
Anna Gorska, University of Warsaw, Warsaw, Poland; and S. P. Malinowski, S. Ł. Błoński, J. Fugal, P. Korczyk, T. A. Kowalewski, and W. Kumala
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We investigate entrainment at the top of cumulus and stratocumulus clouds in a laboratory cloud chamber. We use ultrasonic droplet generators to fill a bottom of the chamber with a layer of cloudy air of a depth of ~50 cm. Mixing of saturated moist air (T~22oC) with dry air from above (T~22oC, RH~35%) results in evaporative cooling which triggers downward convection. In effect, we obtain a temperature inversion capping a well mixed cloud layer below. In these conditions, we force an updraft in the central part of the chamber. A strong updraft penetrating the inversion serves as an analog of an overshooting cumulus cloud, while a weak updraft, diverging below the inversion mimics stratocumulus. The Particle Image Velocimetry technique, where cloud droplets serve as tracers, allows us to retrieve 2D velocity fields in a plane cut through a region of interest. We present the differences in the flow patterns at the top of cumulus and stratocumulus resulting in differences in entraining structures - in particular in sizes and rotations of entraining eddies. We calculate entrainment rates and characterize turbulence in both cases.
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